In the recovery of oil from oil-bearing subterranean formations, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the formation. A variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean formations. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into the formation. As the water moves through the formation, it acts to drive oil therein toward a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the relative mobilities of the oil in the formation and injected water influence the amount of oil recovered by waterflooding. Viscosifiers such as polymeric thickeners are commonly added to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood. Examples of such polymeric thickeners include the polyacrylamides and partially hydrolyzed polyacrylamides. See, for example, U.S. Pat. Nos. 2,827,964; 3,039,529; 3,853,802; 4,249,608; 4,326,969; and 4,409,110. Examples of commercially available hydrolyzed polyacrylamides that are useful as thickeners for waterflooding are the "Pusher" chemicals marketed by the Dow Chemical Company.
A significant problem with respect to many of the commercially available polymeric thickeners relates to their lack of resistance to attack by microorganisms that grow in the floodwater and possibly in the subterranean formation. These microorganisms use the polymeric thickeners as food and thereby destroy their useful properties. Thus far the solution to this problem has been to add a biocide additive to the floodwater to prevent the growth of such microorganisms. Examples of such additives include formaldehyde and glutaraldehyde. The patent literature suggests other additives can also be used. For example, U.S. Pat. Nos. 2,205,558, 3,394,195, and 3,407,204 disclose the use of maleimides; U.S. Pat. No. 3,562,157 discloses the use of malealdehyde; and U.S. Pat. No. 4,049,559 discloses the use of certain di-bicyclo or heptyl or di-bicyclo and heptenyl polyamines. These biocide additives are, however, usually too costly to be used economically. They can also cause plugging in the well if they are not thoroughly mixed in the floodwater. A critical problem with the use of these biocide additives relates to the fact that they tend to separate from the polymeric thickener as they advance through the formation and thus the effectiveness of the additive in protecting the thickener is reduced or lost. The reason for such separation is believed to be due to the fact that the rate of adherence to the rock in the formation for such biocide additives is different than for the polymeric thickeners and thus the rate of advance through the formation for each is different.
An additional problem with the use of many of the commercially available thickeners relates to providing a thickener that is stable and effective in environments characterized by temperatures of from about 15.degree. C. to about 120.degree. C. and above, high pressures (e.g., up to about 4000 psi), high concentrations of divalent or trivalent metal ions such as calcium, magnesium, boron, barium, iron, etc. (e.g., up to 3000 ppm or more and in some instances as high as 10,000 or 20,000 ppm), high salinity (e.g., total dissolved salts (TDS) levels of up to about 200,000 ppm), and low or high pH (e.g., pH as low as about 4 and as high as about 12). Some of these thickeners undergo a marked decrease in viscosity at relatively high temperatures and therefore are not sufficiently thermally stable for use in oil fields having relatively high temperatures. Many are also relatively difficult to inject, have a relatively low tolerance to divalent and/or trivalent metal ions, and/or have poor shear stability.
It would be advantageous to provide a polymeric thickener that would, in itself, be biologically resistant. By providing such a polymer, the problem of separation of the polymer from any biocide additive would be inherently overcome. It would also be advantageous if this thickener could be effectively used in waterflooding operations wherein the polymer would be exposed to high temperatures, high salinities, high pressures, high concentrations of divalent and/or trivalent metal ions, and/or high or low pH.